Polyimide finds a wide spectrum of applications in the electronic material industry including protecting materials, insulation materials, color filters, etc. for liquid crystal displays and semiconductors by virtue of its high mechanical strength, thermal resistance, and solvent resistance. In addition, expectation has recently been made of the use of polyimide as a material for optical telecommunication and as a substrate for mobile phones.
With the recent advances in these industries, materials having advanced properties are increasingly demanded. For example, not only are mechanical properties such as thermal resistance and solvent resistance needed, but also functions according to uses, such as transparency, etc., are requested in polyimide for use in this field.
Appearing with a dark amber color, wholly aromatic polyimides for general purposes, which can be obtained by the polycondensation of aromatic tetracarboxylic dianhydride with aromatic diamine, cannot be applied where high transparency is needed. In addition, since wholly aromatic polyimide is insoluble in organic solvents, its precursor polyamic acid is, in practice, employed before film formation through thermal ring-closing dehydration.
One strategy for achieving transparency is known, wherein aliphatic tetracarboxylic dianhydride is polycondensed with aromatic diamine polyimide to give a polyimide precursor, followed by conversion into polyimide that is relatively colorless, and highly transparent (Japanese Patent Unexamined Application Publication Nos. Hei 2-24294 and Sho 58-208322).
However, the polyamic acids and polyimides that are based on unsubstituted aliphatic tetracarboxylic dianhydride are almost insoluble in general organic solvents, and soluble only in polar organic solvents with high boiling points. In this context, a high temperature is employed upon the formation of the film so as to remove the solvent, exerting undesirable effects on other organic materials of organic EL devices.
In recent years, a polyimide prepared from the monomer 1,2,3,4-cyclopentane tetracarboxylic dianhydride (hereinafter referred to as “CPDA”) has been suggested for use as a gas barrier film of organic electroluminescence (hereinafter referred to as “organic EL”) (Japanese Patent Unexamined Application Publication No. 2006-232960).
However, this polyimide needs to be improved in thermal resistance and is not of sufficient solubility in organic solvents in addition to having a low degree of polymerization.